8. Why should schoolroom windows be all on one side and reach to the ceiling? 9. What is the relation between the size of an image and its distance from the aperture forming it? Can you prove this by geometry? 10. What are silhouettes and how are they produced? (2) PHOTOMETRY AND THE LAW OF REFLECTION =358. Photometry.=--It is desirable at times to compare the intensities of illumination produced by light from different sources. This is done to determine the _relative cost or effectiveness_ of various illuminants such as candles, kerosene and gas lamps, and electric lights The process of determining the relative intensity of lights or lamps is called photometry. (_Photos_ = light.) The unit for measuring the power of light is called a _candle power_. It is the light produced by a sperm candle burning 120 grains per hour. An ordinary gas light burns 5 or more cubic feet of gas per hour and yields from 15 to 25 candle power. A Welsbach gas lamp, consuming 3 cu. ft. per hour, produces 50 to 100 candle power. Instead of using candles, for practical photometry, incandescent lamps standardized by the Bureau of Standards are used for testing or calibration purposes. It is necessary to distinguish between the intensity of a _luminous_ body, _i.e._, as a source of light, and the _intensity_ of _illumination_ upon some surface produced by a light. It is considered that two sources of light are of _equal intensity_ if they produce equal illumination at equal distances. =359. Law of Intensity of Light.=--A device for measuring the candle power of a light is called a _photometer_. Its use is based upon the _law of intensity of light_. _The intensity of illumination of a surface is inversely proportional to the square of its distance from the source of light._ This relation is similar to that existing between the intensity of a sound and the distance from its source. The following device illustrates the truth of this law in a simple manner. [Illustration: FIG. 350.--The light spreads over four times the area at twice the distance.] Cut a hole 1 in. square in a large sheet of cardboard (_K_) and place the card in an upright position 1 meter from an arc light or other _point source_ of light (_L_). Now rule inch squares upon another card (_M_) and place it parallel to the first card and 2 meters from it. (See Fig. 350.) The light that passed through the hole of 1 sq. in. at a distance of 1 meter is spread over 4 sq. in. at a distance of 2 meters. Therefore, the intensity of illumination on each square inch of _M_ is one-fourth that upon the surface of _K_. If _M_ is placed 3 meters from the light, 9 sq. in. are illuminated, or the intensity is one-ninth that at 1 meter distance. [Illustration: FIG. 351.--The Bunsen photometer.]